Psychology has its origins in the desire of Mankind to know itself. Some people who have made significant contributions to psychology are as follows: Socrates (470-399) who proposed knowing thyself; Democritus (400 BCE), who suggested that behavior is the body andthe mind interacting; Aristotle (384-322 BCE), who argued that human behavior is subject to its own natural laws; Gustav Theodor Fechner (1801-1887), who wrote Elements of Psychophysics; Wilhelm Wundt (1832-1920), who founded structuralism; William James (1842-1910), who founded functionalism; Sigmund Freud (1856-1939) who developed psychoanalysis; John Broadus Watson (1878-1958); Max Wertheimer (1880-1943), Kurt Koffka (1886-1941), and Wolfgang Köhler (1887-1967), who founded Gestalt psychology; and B. F. Skinner (1904-1990), who contributed to behaviorism.

Samples and populations are two different things: a sample is a piece of a population pie, but not just any piece — however small, it must show whatthe pie population is made of. It cannot just be the crust or just filling; it’s a slice that must accurately represent the whole.

A population is a whole pie, or a whole group of people, that researchers are interested in and are willing to take a sample from. The sample drawn can be taken randomly (where every member has an equal probability of being represented) or it can be stratified (so that all subgroups are proportionately represented). An issue with some samples is when people volunteer — rather than be selected randomly or for stratification purposes — to participate. They can be a source of volunteer bias because the very act of volunteering means that they are different from those who do not.

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